This invention relates to materials used to polish SiO2-based substrates and specifically to abrasive particles used in such materials.
The most widely used abrasive for polishing SiO2 based materials, such as glass or quartz surfaces is ceria and this is because it gives the highest polishing rate combined with the best surface finish among all the alternative abrasives materials that have been tried such as silica, iron oxide, zirconia and various forms of alumina. However it is also very expensive when supplied as a pure material. To mitigate this factor, ceria has been used in admixture with other abrasives such as oxides of silicon, aluminum, rare earth metals or calcium. This saves money but carries a penalty since, in general terms and in the context of glass polishing, the higher the impurity level, the longer it takes to remove the required amount of surface and the worse the surface finish. In part this is because ceria operates on glass in a chemical-mechanical polishing manner which is not available with the other abrasive oxides. Ceria, with its properties largely dependent upon its defect structure, especially shows affinity to SiO2 which is the basis for the chemical-mechanical polishing effect. It is also softer than SiO2 or SiO2-based glass, which prevent the surface from rough scratches. Thus commercial ceria polishing compositions may contain only 40 to 80% of ceria but the use of such materials is confined to polishing conventional glasses used for mirrors, TV screen surfaces, ordinary optical lenses and so on where the highest finish standards are not so critical. With the maximum chemical-mechanical effect, pure ceria is however still used for the most critical high precision surfaces.
There is therefore a need to supply high quality ceria polishing compositions at a more affordable price that are still adapted for use in producing very high precision polished glass surfaces.
The present invention provides abrasive particles which comprise a ceramic core with a coating of ceria deposited thereon such that the ceria represents for 2 to 30%, and preferably from 3 to 15% of the weight of the particles. In such a core-shell structure, the inert core will provide mechanical action while the shell give chemical action during the polishing.
The ceramic core can be any ceramic oxide but is preferably one on which ceria can readily precipitate and adhere as a thin layer. The interface between the core and shell ceria should be strong enough to ensure that the shell will not easily come off during processing and abrasion action in the course of polishing a substrate. Preferably, not only physical attachment on the cores but strong chemical bonding exists between the CeO2 coating and cores.
The particle sizes are preferably those most useful in putting a polished surface on glass. Most preferably the particles do not contain a large volume of particles that are significantly larger than the average. Therefore with the particle sizes herein being the volume average particle size as measured by a Microtrac technique, the d90 particle size should not be more than an order of magnitude larger than the d50 particle size.
The actual size of the coated particles is determined by the application intended and the practicalities of the process. Large sized particles give higher material removal rate but normally worse surface finishing than the small sized particles. Since a major objective is to reduce the cost of the polishing operation, the use of very fine ceramic core particles will lead to a high surface area and therefore a large amount of ceria will be required to place a coat thereon. In addition the coating process becomes more difficult to carry out. As a practical matter therefore the coated particles are commonly built on 20 to 2000 nanometer core particles and the shell deposited thereon has a thickness, estimate from the increase in weight of the particles, of from 1 to 20 and preferably from 2 to 10 nanometers.
The invention further provides a polishing formulation comprising an abrasive powder as described above dispersed in a carrier medium. The formulation can optionally also contain an adjuvant which aids in the dispersion of debris resulting from the polishing action such that it can be removed from the surface in suspension in the dispersion medium.
The invention further provides a method making abrasive particles comprising a ceramic shell with a surface layer of ceria which comprises dispersing a ceramic powder in a dispersion medium such as water which contains in solution a ceria precursor and treating the dispersion to separate a powder comprising ceramic powder particles having a surface layer of ceria deposited thereon.
The preferred substrate ceramic materials are silica, silicon carbide and alumina. The silica option is more preferably fumed silica since this is readily available in the form of a very fine powder with relatively uniform particle sizes. It is also quite insoluble in water which is the preferred dispersion medium for the production of polishing slurries providing the pH is not excessively acidic or basic. The silica option can also be fused silica powder, which has larger size (xcx9c1-2 um) or colloidal silica, which has smaller size (10 nmxcx9c100 nm) as compared with fumed silica. The alumina option can be particles of alpha alumina, gamma alumina, amorphous alumina or boehmite. Silicon Carbide can be in either the alpha or beta crystalline phase, but with the surface oxidized so that ceria can be more adherent than on SiC itself. References to xe2x80x9csilicon carbidexe2x80x9d in the following should be understood to refer to such surface-oxidized materials.
A polishing slurry containing the novel powder according to the invention is preferably water-based and may contain water-soluble detergent materials such as phosphates, as long as no inhibiting effect upon the contact of ceria to the substrate surface. The pH of the polishing formulation can be kept from 3 to 11. Normally, higher pH leads to higher hydration rate or dissolution rate of SiO2 which benefit the polishing action. However, low pH might be preferred in some cases where good stability of the slurry is required, as ceria can be better dispersed in acidic solution and the slurry will have longer shelf time. In either the cases, the isoelectric point (IEP) of ceria, normally around pH 7, should be avoided.
The production of the ceria-coated particles is preferably accomplished by a solution process. First, ceramic particles need to be mixed with water to make a good and stable dispersion. To the solution, water soluble cerium salts can be added before or after the pH is raised to close to 10 at which point, cerium salts will completely precipitate as oxide or hydroxide. If addition occurs before the pH is raised, a mixture of cerium salt with ceramic particles in water will typically have an acid pH value and this can be adjusted by addition of a base that is effective to raise the pH, typically to about 10. At this pH the cerium salt deposits on the ceramic particles, probably in the form of the hydrated oxide. A straightforward way is to add NH4OH to the solution containing the cerium salts under vigorous stirring until a pH of 10 is reached. Alternatively a basic chemical such as urea can be dissolved in the solution, followed by decomposition of urea into ammonia by heating the solution to an elevated temperature, preferably from about 75 to 90xc2x0 C. and holding the solution at the temperature until ceria deposition is complete. In that way, pH is raised in-situ, and there is no abrupt local increase of pH in a local region as would occur when ammonia is added. Cerium salts can also be added after the pH is raised. For example an aqueous dispersion of ceramic particles, such as SiO2, can be first mixed with ammonia to a high pH of about 10, and an aqueous solution of a cerium salt can then be added drop-wise to the solution while under stirring. As the salt solution is added the pH falls so a certain amount of aqueous ammonia needs to be added to keep the solution around pH 10. Thereafter the particles can be separated by sedimentation or by the use of a centrifuge and fired at from 600 to 1000xc2x0 C. to form the ceria as a thin shell on the surfaces of the ceramic particles. This shell can represent any desired percentage of the weight of the powder, such as from 3 to 25% and most preferably from 3 to 15% of the weight of the particles.
The precursors for the ceria can be any cerium salts that are water soluble, such as Ce(NO3)3.6H2O, Ce(SO4)2.4H2O, (NH4)2.Ce(NO3)6 and the like. It is also possible to use a commercial CeO2 sol, (for example Nyacol).
Production of Core-Shell Structured Particles
A convenient way of producing the core-shell ceramic particles of the invention is exemplified by the following Examples.